Kennedy was targeting Inhofe for a letter the Oklahoma Republican sent last week to EPA Administrator Lisa Jackson that questions the Obama administration's fuel economy and air pollution policies.

Inhofe spokesman Matt Dempsey said Kennedy's tweet crosses the line, especially in the wake of Limbaugh’s apology to a Georgetown University student after calling her a ‘prostitute’ during an on-air broadcast.

"It's hard to believe that Robert Kennedy Jr. would choose to use such language — especially this week," Dempsey said. "I bet he now regrets it."

Kennedy told POLITICO that he stands by his tweet.

"Well, I think it's an opportunity for Americans to compare the relative moral positions of the two acts in question and which is more harmful to our country,” he said. “In Sen. Inhofe's case, here's a U.S. senator whose job is to serve the American public who has made a clear and unambiguous choice to serve the moneyed interests of oil companies.”

“The context with which Mr. Limbaugh used it was wrong and immoral,” Kennedy Jr. added. “But it’s not immoral in using it to apply to a politician who is selling his office.”

A spokesman at the Natural Resources Defense Council, where Kennedy is a senior attorney, declined comment.

This article first appeared on POLITICO Pro at 7:57 p.m. on March 6, 2012.